Color reproduction apparatus renders an image from color signals representing a color image description. The input to such a color reproduction apparatus is an array of color signals (sets of input code values) communicating the desired colors of a corresponding array of picture elements in the image to be rendered.
The reproduction apparatus causes cyan, magenta and yellow, and sometimes black, dyes to be laid down on a receiving medium in response to color signals (its inherent drive code values) applied to its dye marking or producing devices.
A goal is to make the renderings match an original intent as nearly as possible. In order to do so, a transform is employed which maps the received input code values, normally in RGB space, to the reproduction apparatus' inherent drive code values, so that the system faithfully reproduces the color image original. Often the transform is implemented by employing a set of three or four one-dimensional calibration lookup tables. The process of deriving such a transform is called system color calibration.
Such apparatus requires re-calibration from time to time due to changes in the apparatus' behavior, as might be affected by the state of processor chemicals if any are used, color marking system drifts, and changes in receiver characteristics. Differences in receiver characteristics might be caused by different recipes, variations in receivers from one coating event to another, and different surface finishes (i.e., glossy versus matte finishes) as are commonly encountered using photographic receivers.
Calibration of a color reproduction apparatus begins by specifying color aim curves. A color aim curves are a set of three or four separate curves, one for each of the red, green blue and possibly black channels and each specifies the desired density output as a function of the corresponding input code value. It is intended that the three (or four) separate curves are to be used in concert and is designed to sample the entire density range. It is commonplace that when the input code values for red, green and blue channels are equal, that a visually neutral gray is to be rendered. The color aim curve defines both the definition of neutral in terms of the relationship between measured densities and the relation between (common) input code values and output densities. The entries of the table define a relationship of density as a function of input code value, or:Density=f(iCV) It is further commonplace to develop three such functions for a 3-color printer, one for each color. Thus, the family of functions can be expressed as:DensityRed=fred(iCVr), DensityGreen=fgreen(iCVg), and DensityBlue=fblue(iCVb). These equations are used to find the correct drive code values for each entry in the three calibration lookup tables.
Often, aim curves are linear. A truly neutral density (gray) calibration target (FIG. 1) contains a series of aim neutral (gray) density patches, which are encoded as grays ranging from white to black. The input code values in the target's description are passed through existing calibration lookup tables to remap them into drive code values that the reproduction apparatus will actually use to make a print. If the reproduction apparatus is in calibration, the resultant print will have the correct aim densities. Mathematical transforms may be employed in lieu of lookup tables. The resulting densities of the rendered patches are measured using an optical instrument such as a densitometer; and compared to the aim densities. If any of the patch density values are different than aim, being either too light or dark, or possess a non-neutral hue, then new sets of calibration lookup table values or transforms are derived using a mathematical method such as regression so as to increase or decrease the density of the corresponding color.
The limitation in this process is that the red density is not just a function of the cyan drive code value. Rather, it is a function of the drive code values of all the other colors. Albeit, the red density has a strong dependency on the cyan drive code value and a weaker dependency on the other drive code values. This being the case, if one adjusts, say, the magenta drive code value to make a correction to the green density, then the red density will change even though the cyan code values do not change. Similarly, changes to the cyan code values will produce changes to the green and blue densities. Although this known calibration process usually works because the cross-dependencies are small, it is often necessary to conduct a series of successive calibrations, each using the previous output lookup tables, in order to achieve a final calibration within specification.